1. Field of the Invention
This invention relates to erasable optical disks of the type in which information is recorded, reproduced and/or erased by the use of laser beams and more particularly, to an improvement in optically transparent substrates of cured resin products useful for the optical disks. The invention also relates to the cured resin products of reactive compound compositions.
2. Description of the Prior Art
As is well known in the art, a typical optical recording disk includes an optically transparent substrate having a spiral groove or concentric grooves on one side thereof and a recording film formed on the grooved surface by evaporation or sputtering. When a laser beam is irradiated on the recording film through a groove-free side according to information signals, a phase change takes place in the film where irradiated. The information signals are recorded as variations in reflectance or transmittance caused by the phase change. The thus recorded information signals can be reproduced by application of a laser beam of a different energy level to the recording film. Similarly, the information signals can be erased by applying a laser beam to the recording film so that the phase change is reversely caused until the reflectance or other characteristics are returned to an original state. For instance, if information signals are recorded by converting the recording film from amorphous to crystalline states at portions irradiated with a laser beam, the signals are erased by conversion to the amorphous state.
In this type of optical disk, a glass substrate is predominantly used as the optically transparent substrate. In some cases, thermoplastic resin substrates are used for this purpose, including a polymethyl methacrylate substrate and a polycarbonate substrate. The polymethyl methacrylate or polycarbonate substrate is advantageous in that it can be fabricated using existing injection molders, thus leading to a high productivity. However, because of the high molecular weight of the resin, it is difficult to completely remove foreign matters from these polymers with a relatively high percent defective. In addition, the birefringence of the substrate cannot be reduced because of the melt flow of polymer molecules during the molding. Moreover, the injection-molded resin substrate has the serious problem that it is not resistant to heat and is not suitable for use as an erasable optical disk which utilizes the phase change of a recording film from crystalline to amorphous states, and vice versa, by application of laser beams. More particularly, when laser beams of different energy levels are repeatedly applied to a recording film formed, for example, on a polycarbonate substrate for repeating recording, reproducing and erasing cycles, the polycarbonate substrate undergoes thermal deformation after only several to several tens repetition cycles. This results in formation of cracks or pinholes in the recording film, making it impossible to continue the recording, reproducing and erasing operations. This is true of a polymethyl methacrylate substrate. Accordingly, the optical disks using these resin substrates cannot be reliably used as an erasable disk, but may be used only as a write-once optical disk.
On the other hand, a photopolymer groove transfer method is known in which a grooved photopolymer is transferred on a glass substrate or an epoxy resin substrate. However, the disks obtained by this method have the drawback that the photopolymer is not resistant to heat. The glass substrate is expensive and presents a cost problem. The epoxy resin substrate is disadvantageous in that it takes a relatively long time for curing, resulting in a poor productivity, and in that the releasing property of the epoxy resin is poor.
In order to set up a standard for the heat resistance of a recording film, we made a computer analysis of an optical disk, with the result that the recording film was heated to a temperature over 200.degree. C. by application of laser beams.
To solve the problem involved in the heat resistance, attempts have been made to form a thermal insulating film of an inorganic material between a resin substrate and a recording film. However, optimum conditions of forming an inorganic film on a polycarbonate or polymethyl methacrylate substrate have not been established yet with respect to a type of apparatus, a technique and a type of inorganic material. In addition, because existing apparatus might not be used for this purpose, plant costs would become high with an increasing cost of the resultant optical disk. Thus, this type of optical disk has not been in use yet.
Accordingly, there is a high demand for an optically transparent resin substrate which has a high heat resistance without the necessity of any inorganic thermal insulating film and which has a reduced birefrigence and can be fabricated while easily removing foreign matters from starting materials.